How do I know there's only one universe? The same reason I know time travel to the past is impossible. If it were possible, we'd already see evidence of it because somebody would have already been back here (even if they weren't supposed to).
How does this relate to multiple universes? Well, the only reason we have a multiverse theory is because the possibility of our existence is so remote, scientists realized it was illogical to believe life could spring up into existence in one universe. But, if you posit an infinite number of universes, well, then... ANYTHING is possible!
And therein is your problem. It's kinda like the grandpa paradox for time travel. The one thing that destroys this entire multiverse stupidity is that in one of those infinite universes, someone has discovered not only how to travel to other universes, but how to destroy them all. And since we're still here, well...
There's only one universe, and we're in it. Any other theory is simply a fairy tale.